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Results for “first principles calculation”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Hybrid molecular beam epitaxy of germanium-based oxides

Abstract Germanium-based oxides such as rutile GeO 2 are garnering attention owing to their wide band gaps and the prospects of ambipolar doping for application in high-power devices. Here, we present the use of germanium tetraisopropoxide (GTIP), a metal-organic chemical precursor, as a source of germanium for the demonstration of hybrid molecular beam epitaxy for germanium-containing compounds. We use Sn 1- x Ge x O 2 and SrSn 1- x Ge x O 3 as model systems to demonstrate our synthesis method. A combination of high-resolution X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy confirms the successful growth of epitaxial rutile Sn 1- x Ge x O 2 on TiO 2 (001) substrates up to x = 0.54 and coherent perovskite SrSn 1- x Ge x O 3 on GdScO 3 (110) substrates up to x = 0.16. Characterization and first-principles calculations corroborate that germanium occupies the tin site, as opposed to the strontium site. These findings confirm the viability of the GTIP precursor for the growth of germanium-containing oxides by hybrid molecular beam epitaxy, thus providing a promising route to high-quality perovskite germanate films.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dynamics and resilience of the unconventional charge density wave in ScV 6 Sn 6 bilayer kagome metal

Long-range electronic ordering descending from a metallic parent state constitutes a rich playground to study the interplay of structural and electronic degrees of freedom. In this framework, kagome metals are in the most interesting regime where both phonon and electronically mediated couplings are significant. Several of these systems undergo a charge density wave transition. However, to date, the origin and the main driving force behind this charge order is elusive. Here, we use the kagome metal ScV 6 Sn 6 as a platform to investigate this problem, since it features both a kagome-derived nested Fermi surface and van-Hove singularities near the Fermi level, and a charge-ordered phase that strongly affects its physical properties. By combining time-resolved reflectivity, first principles calculations and photo-emission experiments, we identify the structural degrees of freedom to play a fundamental role in the stabilization of charge order, indicating that ScV 6 Sn 6 features an instance of charge order predominantly originating from phonons.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Candidate spin-liquid ground state in CsNdSe 2 with an effective spin-1/2 triangular lattice

Rare-earth-based triangular lattice materials are extremely attractive for studying unconventional magnetism. Here, we report the magnetic properties of layered CsNdSe 2 based on direct current (DC) and alternating current (AC) susceptibility measurements down to 0.04 K. While the AC susceptibility at the zero DC field shows a broad hump below 0.5 K, there is no sign of any long-range magnetic ordering. Quantitative analysis of the DC magnetic susceptibility gives the negative Curie-Weiss (CW) temperature θ CW < 0 in all directions, indicating antiferromagnetic interaction between Nd ions. Of particular interest is the low temperature magnetic susceptibility, which reflects the effective spin-1/2 state with $\theta^a_{\text {cw}}/\theta^c_{\text {cw}}$ > 3. The estimated exchange interactions are J a /k B = 1.42 K (in-plane) and J c /k B = 0.44 K (out-of-plane), pointing to the anisotropic magnetism. First-principles calculations that include spin-orbit coupling and Coulomb correlations reveal multiple states with zero net magnetization for CsNdSe 2 . Both experiment and simulation strongly suggest CsNdSe 2 has the spin liquid ground state with effective spin-1/2. Application of a magnetic field can induce long-range antiferromagnetic ordering with the maximum transition temperature around 0.3 K, in further support of the zero-field spin liquid state.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Signatures of Kramers-Weyl fermions in the charge density wave material (TaSe 4 ) 2 I

The quasi-one-dimensional charge density wave (CDW) material (TaSe 4 ) 2 I has been recently predicted to host Kramers-Weyl (KW) fermions which should exist in the vicinity of high symmetry points in the Brillouin zone in chiral materials with strong spin-orbit coupling. However, direct spectroscopic evidence of KW fermions is limited. Here we use helicity-dependent laser-based angle-resolved photoemission spectroscopy (ARPES) in conjunction with tight-binding and first-principles calculations to identify KW fermions in (TaSe 4 ) 2 I. We find that topological and symmetry considerations place distinct constraints on the (pseudo-) spin texture and the observed spectra around a KW node. Our findings highlight the unique topological nature of (TaSe 4 ) 2 I and provide a pathway for identifying KW fermions in other chiral materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Catalytic deconstruction of organic additive-containing plastics

Plastics waste ends up in landfills, oceans, and incinerators, posing major environmental and human health threats. Catalytic deconstruction is emerging as a key technological solution to handle complex plastics and has successfully converted virgin polymers into various products. Here, we investigate the resilience of chemical deconstruction technologies to organic additives, which are ubiquitous in plastics. We study catalyst-additive interactions experimentally and via first-principles calculations for plastics additives representative of entire classes. We reveal two deactivation mechanisms and demonstrate that many recently developed catalysts are inadequate for polyolefin conversion due to poisoning caused by the strong adsorption of most additives or their small fragments. Furthermore, we also identify conditions and catalysts that can circumvent the challenge of deconstruction in the presence of additives challenge.

10 SYNTHETIC FUELS↗
DOE OSTI2024

Emergence of exchange bias in van der Waals magnetic alloy CrxPt1−xTe2

Abstract Cr x Pt 1− x Te 2 is a recently developed van der Waals magnetic alloy noted for its stability under ambient conditions. Here, we report the emergence of an exchange bias effect in Cr x Pt 1− x Te 2 , without typical exchange bias sources such as an adjacent antiferromagnetic layer. We find that the exchange bias is present for x = 0.45 and absent for x = 0.35, which is correlated to the presence of a Cr modulation where the Cr concentration alternates each vdW layer (modulation period of 2 layers) for x ≥ 0.4. We perform Monte Carlo simulations utilizing exchange parameters from first-principles calculations, which recreate the exchange bias in hysteresis loops of Cr 0.45 Pt 0.55 Te 2 . From our simulations, we infer the source of exchange bias to be magnetic moments locked into free energy minima that resist magnetization reversal. This work presents a way to introduce desirable magnetic properties to van der Waals magnets.

Bailey-Crandell, Ryan↗
DOE OSTI2020

Synthesis, transport properties and electronic structure of p-type Cu 1+x Mn 2–x InTe 4 ( x = 0, 0.2, 0.3)

The synthesis, electronic structure and temperature dependent transport properties of polycrystalline Cu 1+x Mn 2–x InTe 4 (x = 0, 0.2, 0.3) are reported for the first time. These quaternary chalcogenides were synthesized by direct reaction of the elements, followed by solid state annealing and hot press densification. The thermal conductivity is low for all specimens and intrinsic to the material system. Furthermore, the off-stoichiometry specimens illustrate the sensitivity of the transport properties to stoichiometry, with a greater than two-orders-of magnitude increase in carrier concentration with increased Cu content. First principles calculations of the electronic structure are also reported, and are in agreement with the experimental data. Here, this fundamental investigation shows the potential towards further optimization of the electrical properties that, in addition to the intrinsically low thermal conductivity, provides a basis for further research into the viability of this material system for potential energy-related applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Giant spin Seebeck effect through an interface organic semiconductor

Interfacing an organic semiconductor C 60 with a non-magnetic metallic thin film (Cu or Pt) has created a novel heterostructure that is ferromagnetic at ambient temperature, while its interface with a magnetic metal (Fe or Co) can tune the anisotropic magnetic surface property of the material. Here, we demonstrate that sandwiching C60 in between a magnetic insulator (Y 3 Fe 5 O 12 :YIG) and a non-magnetic, strong spin–orbit metal (Pt) promotes highly efficient spin current transport via the thermally driven spin Seebeck effect (SSE). Experiments and first principles calculations consistently show that the presence of C 60 reduces significantly the conductivity mismatch between YIG and Pt and the surface perpendicular magnetic anisotropy of YIG, giving rise to enhanced spin mixing conductance across YIG/C 60 /Pt interfaces. As a result, a 600% increase in the SSE voltage (V LSSE ) has been realized in YIG/C 60 /Pt relative to YIG/Pt. Temperature-dependent SSE voltage measurements on YIG/C 60 /Pt with varying C 60 layer thicknesses also show an exponential increase in V LSSE at low temperatures below 200 K, resembling the temperature evolution of spin diffusion length of C 60 . Our report emphasizes the important roles of the magnetic anisotropy and the spin diffusion length of the intermediate layer in the SSE in YIG/C 60 /Pt structures, providing a new pathway for developing novel spin-caloric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Crystallographic structure and crystal field parameters in the [An IV (DPA) 3 ] 2- series, An = Th, U, Np, Pu

The [An IV (DPA) 3 ] 2- series with An = Th, U, Np, Pu has been synthesized and characterized using SC-XRD and vibrational spectroscopy. First principles calculations were performed, the total electron density is analyzed using the Quantum Theory of Atoms in Molecules. Crystal field parameters and strength parameters are deduced following a previous work on the Ln III analog series e.g. [J. Jung et al., Chem. – Eur. J., 2019, 25, 15112]. The trends in the parameters along the series are compared to the Ln III complexes. They evidence larger covalent interactions and larger J mixing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

π-Orbital Yin–Yang Kagome bands in anilato-based metal–organic frameworks

π-Orbital bonding plays an important role not only in traditional molecular science and solid-state chemistry but also in modern quantum physics and materials, such as the relativistic Dirac states formed by bonding and antibonding π-bands in graphene. In this work, we disclose an interesting manifestation of π-orbitals in forming the Yin–Yang Kagome bands, which host potentially a range of exotic quantum phenomena. Based on first-principles calculations and tight-binding orbital analyses, we show that the frontier π 2 - and π 3 -orbitals in anilato-based metal–organic frameworks form concurrently a conduction and valence set of Kagome bands, respectively, but with opposite signs of lattice hopping to constitute a pair of enantiomorphic Yin and Yang Kagome bands, as recently proposed in a diatomic Kagome lattice. The twisted configuration of neighboring benzene-derived organic ligands bridged by an octahedrally O-coordinated metal ion is found to play a critical role in creating the opposite sign of lattice hopping for the π 2 - versus π 3 -orbitals. Our finding affords a new material platform to study π-orbital originated quantum chemistry and physics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

First-principles studies of strongly correlated states in defect spin qubits in diamond

Using a recently developed quantum embedding theory, we present first-principles calculations of strongly correlated states of spin defects in diamond. Using this theory, effective Hamiltonians are constructed, which can be solved by classical and quantum computers; the latter promise a much more favorable scaling as a function of system size than the former. In particular, we report a study on the neutral group-IV vacancy complexes in diamond, and we discuss their strongly correlated spin-singlet and spin-triplet excited states. Finally, our results provide valuable predictions for experiments aimed at optical manipulation of these defects for quantum information technology applications.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Switchable electronic and enhanced magnetic properties of CrI 3 edges

Owing to its novel electronic and magnetic properties, two-dimensional CrI 3 has great potential in the application of spintronic devices. However, as an inevitable line defect, the properties of the edges of CrI 3 remain elusive. Here, via first-principles calculations with spin–orbit coupling, we investigated the thermodynamic stabilities, electronic and magnetic properties of thirteen CrI 3 edges with different structures. We showed that zigzag edges are more stable than armchair edges, and a CrI 3 nanoribbon can be either metallic or insulating depending on its chemical growth conditions. Here, the edge stability and associated electronic properties can be understood in terms of the octahedron ligand field and electron counting model. In most cases, both the magnetic moment and Curie temperature can be enhanced by edges, which are in startle contrast to the surfaces of three-dimensional ferromagnetic materials, where a magnetic dead layer is often observed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Computational discovery of promising new n-type dopable ABX Zintl thermoelectric materials

Computational prediction of good thermoelectric (TE) performance in several n-type doped Zintl phases, combined with successful experimental realization, has sparked interest in discovering new n-type dopable members of this family of materials. However, most known Zintls are typically only p-type dopable; prior successes in finding n-type Zintl phases have been largely serendipitous. Here in this paper, we go beyond previously synthesized Zintl phases and perform chemical substitutions in known n-type dopable ABX Zintl phases to discover new ones. We use first-principles calculations to predict their stability, potential for TE performance as well as their n-type dopability. Using this approach, we find 17 new ABX Zintl phases in the KSnSb structure type that are predicted to be stable. Several of these newly predicted phases (KSnBi, RbSnBi, NaGeP) are found to exhibit promising n-type TE performance and are n-type dopable. We propose these compounds for further experimental studies, especially KSnBi and RbSnBi, which are both predicted to be good TE materials with high electron concentrations due to self-doping by native defects, when grown under alkali-rich conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

1D topological phases in transition-metal monochalcogenide nanowires

The Su–Schrieffer–Heeger (SSH) model is a prototypical one-dimensional (1D) diatomic lattice model for non-trivial topological phases and topological excitations. In theory, many variations and extensions of the SSH model have been proposed and explored to better understand the novel aspects of topological physics in low dimensions on the nanoscale. However, the outstanding challenge remains to find real nanomaterials with robust structural stability for realizing the 1D topological states. Here, we develop an extended version of the SSH model with multi-atomic bases of four, six and eight atoms and an imposed screw rotation symmetry. Furthermore, based on first-principles calculations, we demonstrate the realization of this model in transition metal monochalcogenide M 6 X 6 (M = Mo and W; X = S, Se and Te) nanowires. The topological features of the doped M 6 X 6 nanowires are confirmed with non-trivial edge modes and e/2 fractional charges, representative of the 1D non-trivial Zak phase. Our finding not only sheds new light on our fundamental understanding of 1D topological physics, but also significantly extends the scope of 1D topological materials that will attract immediate experimental interest, since isolated M 6 X 6 nanowires have already been synthesized in experiments.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Effect of fluorination and Li-excess on the Li migration barrier in Mn-based cathode materials

Disordered rocksalt (DRX) Li-rich transition metal (TM) oxides, especially those based on Mn, are prospective high-energy-density cathode materials for the next generation of Li-ion batteries that use earth abundant metals. Fluorine substitution on the oxygen sublattice has been shown to reduce oxygen redox by lowering the average anion valence and increasing the amount of redox-active TM, and simultaneously improve energy density, average voltage and capacity retention. While these benefits of fluorination are well established, it is not well understood how F affects Li transport and therefore the rate performance of Mn-based DRX cathodes. Herein, we investigate the effects of both F substitution and the accompanying Li-excess on Li migration barriers using first-principles calculations. Finally, we demonstrate that F has a small negative effect on Li migration barriers while Li-excess decreases Li migration barriers. Because fluorination enables more Li-excess, these results do not predict any detrimental impact on Li transport.

25 ENERGY STORAGE↗
DOE OSTI2020

Thermodynamic and kinetic properties of layered-CaCo2O4 for the Ca-ion batteries: a systematic first-principles study

One of the more promising directions in multivalent energy storage is systems based on Ca ion intercalation due to the potential for high voltage and capacity. A major challenge for enabling such a battery is to find cathode materials capable of fast ionic diffusion and reversible insertion of Ca ions. Here, on the basis of first-principles calculations, we have demonstrated that layered CaCo 2 O 4 exhibits favorable thermodynamic and kinetic properties that should enable topotactic Ca ion intercalation reactions. The P3-type layered Ca x Co 2 O 4 (0 < x < 1) with either of space groups of P 1 or P 2 1 / m are stable at multiple Ca concentrations and show a smooth voltage plateau higher than 3 V up to X = 0.5. The energy barriers of the single Ca ion migration are as low as 0.36 eV and 0.27 eV at the dilute and high vacancy concentration limits, respectively. Therefore, although varying the vacancy environments of the diffusing atom influences the migration barriers, they do not exceed 0.6 eV. Stochastic analysis of Ca hopping events performed by ab initio molecular dynamics (AIMD) simulation has shown that the migration barriers are lower than 0.32 eV. Therefore, the Ca diffusivity at room temperature extrapolated from the AIMD results is comparable to Li diffusivity (>10 -10 cm 2 s -1 ) in conventional Li cathode materials, suggesting the feasibility of layered Ca x Co 2 O 4 as multivalent cathode materials. Finally, the structural factors that enable fast diffusion are discussed.

25 ENERGY STORAGE↗